Science and Issues

Chemicals from Agriculture

An important national issue in the United States is the degradation of
water quality from
nonpoint sources
of pollution, including the prevalent use of fertilizers and
pesticides
on agricultural land. The issue is of interest to many residents, water
resource managers, and policymakers across the nation because of the
possible impacts on water uses, such as drinking,
irrigation
, recreation, and sustaining aquatic life.

In sufficient quantities, nutrients from fertilizers encourage abundant
growth of
algae
, which leads to low oxygen in streams and the possibility of fish kills.
Pesticides and
nitrate
are a potential concern for human health if they affect a drinking-water
source or occur where there is recreational use. Elevated concentrations
of nitrate have been associated with
methemoglobinemia
, or "blue baby syndrome" in infants, and stomach disorders,
and some pesticides have been associated with the potential for causing
cancer.

For the protection of drinking water and aquatic life, the U.S.
Environmental Protection Agency (EPA), along with the states, have
established water-quality standards and criteria for some pesticides and
nitrate. These provide widely used benchmarks that serve as starting
points for evaluating potential effects of exposure to these chemicals.

Nationwide Sampling Studies

Nationwide sampling of nitrogen, phosphorus, and more than eighty
pesticides by the U.S. Geological Survey (USGS) from 1992 to 1996
indicated that streams and
groundwater
in agricultural basins almost always contain complex mixtures of
nutrients and pesticides. Concentrations of nitrogen and phosphorus in
streams commonly exceeded levels that can contribute to excessive plant
growth. Average annual concentrations of phosphorus in three-fourths of
seventy-five streams sampled in agricultural areas were greater than the
desired EPA goal for preventing nuisance plant growth in streams.
Nitrate was most prevalent in shallow groundwater (less than 30.5 meters
(100 feet) below land surface) beneath agricultural areas, where about
15 percent of samples collected from 36 different agricultural areas
exceeded the EPA drinking-water standard for nitrate.

Herbicides.

At least one pesticide was found in almost every water and fish sample
collected from forty streams and in over one-half of the more than nine
hundred shallow wells sampled in agricultural areas. A relatively small
number of heavily used chemicals accounted for most detections. The most

Pesticides applied in plant nurseries and greenhouses are less
visible forms of agricultural chemicals than those applied to fields
and orchards. But any form of chemical application can potentially
contaminate water sources if not managed properly.

frequently detected pesticide compounds in streams and shallow
groundwater in agricultural areas were the major
herbicides
atrazine (and its
transformation product
desethylatrazine, or DEA), metolachlor, cyanazine, and alachlor, which
ranked in the top five in national herbicide use for agriculture.
Transformation products of metolachlor, alachlor, and cyanazine would
probably also have been frequently detected if they had been analyzed.

Insecticides.

Compared to herbicides, currently used insecticides were less frequently
found in agricultural streams, and even less in groundwater underlying
agricultural areas. This results from their relatively low application
rates and rapid breakdown in the environment. In contrast, historically
used insecticides still persist in agricultural streams because of their
resistance to breakdown in the environment.

Dichlorodiphenyltrichloroethane, commonly known as DDT, was the most
commonly detected
organochlorine
compound, followed by dieldrin and chlordane. DDT and aldrin (which
breaks down rapidly to dieldrin in the environment) were two of the top
three insecticides used for agriculture in the 1960s. Because of
negative impacts on birds and other species, their uses were restricted
in the 1970s; and yet, more than 20 years later, one or more
sediment-quality guidelines were exceeded at 15 percent of sampled
agricultural sites, and concentrations in whole fish exceeded wildlife
guidelines at 20 percent of sampled sites.

Mixtures of Compounds.

Most samples with a detectable pesticide contained mixtures of
compounds. Nearly half of stream samples collected in agricultural areas
contained five or more pesticides compounds, and about 15 percent
contained more than ten compounds. Nearly one-third of shallow
groundwater samples within agricultural areas had two or more
pesticides. Atrazine, DEA, and metolachlor were the most commonly
detected compounds in mixtures found in agricultural areas.

Water Quality Criteria.

Pesticide concentrations in streams (as annual averages, upon which
drinking-water criteria are based) were generally low,
only exceeding the EPA
maximum contaminant level
for atrazine in one location. Similarly, pesticide concentrations
seldom exceeded the drinking water criteria in wells. Less than one-half
of one percent of the sampled shallow wells in agricultural areas had
concentrations greater than a criterion (one well for atrazine, one for
cyanazine, two for dieldrin, and one for dinoseb).

Geographic Patterns.

Analysis of geographic patterns in pesticide use revealed that
concentrations of herbicides and insecticides in agricultural streams
were highest in those areas of the nation with the greatest agricultural
use. Herbicide concentrations were greatest in central U.S. streams,
where use is most extensive. The direct relationship between chemical
use and chemical concentrations in nearby surface water was demonstrated
in some upper Midwest streams in 1994. After a new herbicide,
acetochlor, partially replaced alachlor, nearby streams quickly showed
increased acetochlor concentrations and decreased alachlor
concentrations. For example, in the White River of Indiana, acetochlor
was commonly detected, reaching a peak concentration of 2 parts per
billion.

Water contamination in agricultural areas is not, however, determined
solely by chemical use. Natural features—
topography
, geology, soil type,
hydrology
, and climate—and land-management practices—tile drainage
and irrigation and conservation strategies—make some areas more
vulnerable to contamination than others. For example, some of the
highest concentrations of nutrients and pesticides were in sand and
gravel
aquifers
or in
karst
formations consisting of carbonate rocks with large fractures, voids,
or conduits. These natural geologic features readily transmit water and
are common in various parts of the United States.

In contrast, groundwater contaminants underlying farmland in parts of
the upper Midwest were barely detectable, despite similar high rates of
chemical use. This is partly because the groundwater is somewhat
shielded from surface infiltration of chemicals by relatively
impermeable and poorly drained soils and glacial till that cover much of
the region. In addition, tile drains and ditches commonly provide quick
pathways for chemical transport to streams, which minimize the downward
movement of contaminants to groundwater.

Seasonal Patterns.

The USGS findings not only documented geographic patterns, but seasonal
patterns as well. In streams that drain agricultural areas throughout
most of the nation, the highest levels of nutrients and pesticides
occurred during spring and summer when recently applied chemicals are
washed away by spring rains, snowmelt, and irrigation. In some parts of
the country, other patterns were found, such as those in the San
Joaquin-Tulare Basins in California where elevated concentrations of the
insecticide diazinon occurred in the winter because of the use of
pesticide sprays on the region's dormant orchards.

Impact to Drinking Water and Aquatic Life.

Although the USGS study results indicate few problems for drinking
water, conclusions must be tempered by several considerations,
including: (1) criteria are not established for many pesticides, and
federal drinking-water standards (administered by the U.S. Environmental
Protection Agency) address only a small number of pesticides; (2)
mixtures and transformation products are not considered; and (3) effects
of seasonal exposure to high concentrations have not been evaluated.
Assessment of the pesticide risks to aquatic life is hampered by

Hooded sprayers are among the improvements to chemical application
methods. In this photograph, the hoods are directing herbicide just
to the areas between rows of grain sorghum.

many of the same issues, but existing water quality criteria were also
more often exceeded. For example, the criteria for atrazine was exceeded
in nearly 40 percent of sampled agricultural streams.

A Historical Perspective

Agricultural chemicals in water must be assessed not only in terms of
scientific findings, but also in the broader context of U.S.
agricultural development, a growing conservation ethic, and economics.
In the past, farmers focussed primarily on production from their fields,
often with little regard to the watershed in which the fields are
located. Agricultural chemicals were seen only as scientific marvels;
they killed the weeds which reduced crop growth and eliminated the
insects that otherwise would have significantly reduced the quality of
their crops. The potential impact on the environment, or for that
matter, human health, simply was not on the average farmer's
mind. Wearing protective clothing while mixing or applying pesticides
was unheard of, and concern about runoff to streams or infiltration to
groundwater was not considered.

Concern about the potential harm from agricultural chemicals began
slowly in the late 1950s and early 1960s and has been growing in both
the
public and the agricultural community since. Today, farmers are
increasingly aware of the complex interrelationships between
agricultural practices and environmental quality. Modern farmers now
consider the timing of agricultural chemical application and irrigation,
the amount and style of pesticide application, specific crop needs, and
local weather conditions in their pesticide and fertilizer use.

In some areas, it was common practice for farmers to apply a heavy load
of fertilizer in the fall, reasoning that it would "be ready when
the plants needed it in the spring," but not realizing that most
of the fertilizer would leach from the soil before the plants began to
grow. It was also common to broadcast pesticides by area-wide spraying,
covering the ground where the targeted plants were located, as well as
ground where they were not.

But today's farmer, with the help of agricultural research,
applies fertilizer and pesticide directly on the crop row, reducing the
chemical needed and reducing the potential for leaching or runoff.
Applying chemicals in a manner so that they do not leach below the root
zone before the plant can utilize them is the key to both crop
production and environmental protection.

Farmers often check the weather conditions in the morning to adjust the
droplet size in the sprayer based on forecasted wind and humidity. It is
not uncommon for today's farmer to make use of satellite and GPS
(global positioning system) technology to tailor the application of
fertilizer and pesticide to the specific crop needs within a single
field. Careful consideration of crop needs and the best method to
deliver an agricultural chemical to its target has often led to a
decrease in chemicals applied per acre, a decrease in the cost to the
farmer, and less of a risk to the environment.

Bibliography

Goss, D. W., and R. D. Wauchope. "The SCS/ARS/CES Pesticide
Properties Database: Using It with Soils Data in a Screening
Procedure." In
Pesticides in the Next Decade: The Challenges Ahead,
ed. D. I. Weigman. Blacksburg, VA: Virginia Water Resources Research
Center (1990):471–493.

RESIDENTIAL USES OF FERTILIZERS AND PESTICIDES

High levels of chemical contamination are not just an agricultural
problem. A U.S. Geological Survey study found that
insecticides—most commonly diazinon, carbaryl, malathion, and
chlorpyrifos—occurred more frequently and usually at higher
concentrations in urban streams than in streams in agricultural areas.

About two-thirds of urban streams sampled in the study had
concentrations of insecticides that exceeded at least one guideline
established to protect aquatic life. In addition, concentrations of
total phosphorus generally were as high in urban streams as in
agricultural streams, commonly exceeding the desired federal goal to
control excessive plant and algae growth.

As in agricultural areas, the types and concentrations of chemicals
found in urban streams are closely linked to the chemicals used, such as
on lawns, gardens, and in public areas. Reducing the amount of chemicals
used and applying these chemicals more efficiently are two effective
ways to reduce contaminant levels in both urban and agricultural
settings.

User Contributions:

pesticides don't usaully kill the bugs we're aiming for so why don't we make stronger pesticides. If we make them to strong won't they get into the soil. we know that they make them so they won't but some time they will kill all the corn or beans.

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